The invention relates to the field of CMOS Schmitt trigger cells, and in particular to a CMOS Schmitt trigger cell with a disable function.
It is common to need to know when a signal crosses a predetermined level. Schmitt triggers are one type of circuit used in such applications. A Schmitt trigger is advantageous for such applications because its output depends on the input signal and on its recent history (i.e., it exhibits hysterisis). Thus, noisy input signals do not cause multiple transitions in the output as the input crosses the trigger point. Rather, the spurious effects of the input must be greater than the threshold difference to cause a transition in the output.
Schmitt trigger cells used in an integrated circuit (IC) typically have an input stage that is a Schmitt trigger and an output buffer stage. In most ICs, the input stage is powered by the I/O voltage, VI/O, and the output buffer stage is powered by the core voltage, Vcore. In this case, in addition to acting as a buffer, the output stage also provides a level shifting function to shift the Schmitt trigger output from the VI/O domain to the Vcore domain.
Standard Schmitt trigger cells consume power under various conditions. For example, Schmitt trigger cells consume power if the input to the Schmitt trigger is allowed to float or if the input is a toggling input. Low power designs, however, require controlled current flow in all transistors. Thus, a Schmitt trigger cell that can be disabled during such conditions would be advantageous for low power designs.
One aspect of the present invention provides for a Schmitt trigger cell having a Schmitt trigger circuit with an input and an output; and an output buffer circuit having an output terminal, and an input that is connected to the output of the Schmitt trigger circuit. A disable transistor is connected to the Schmitt trigger circuit and output buffer circuit such that current flow can be disabled through those transistors of the Schmitt trigger circuit, and the output buffer circuit, under conditions of unknown gate voltages. Examples of such conditions wherein gate voltages are unknown include, but are not limited to, floating input or toggling input. A transistor is also connected to the output buffer circuit such that a known output can be forced onto the output terminal.
Another aspect of the present invention provides a method of disabling a Schmitt trigger cell during unknown gate voltage conditions (e.g., floating or toggling input). Current flow is disabled through those transistors of the Schmitt trigger cell whose gate voltages are unknown due to such input conditions. A known output is also forced onto the output terminal.